In a cellular environment, at any one time, there is usually one serving cell defined as the cell with the base station that an active mobile station is receiving service from so that the mobile station may receive and transmit communication via the serving cell base station. There are a number of surrounding cells that are neighbouring cells. The serving. cell may also be referred to as the cell that the mobile unit is camped on to. In a multicellular environment, there may be cells of different sizes where a number of cells of similar size are located within one larger cell (umbrella cell). The smaller cells within the umbrella cell are called microcells. The umbrella cell may be referred to as a macrocell.
Microcells are created in a dense population of users to allow a greater capacity of users on the cellular system and improve spectral efficiency. The microcells facilitate the reuse of frequencies over a smaller distance. Thus, a mobile unit may be within a microcell as well as an umbrella cell.
Such a two tiered combined cell architecture includes an overlay macrocell layer, comprising of at least one macrocell and an underlay microcell layer, comprising of a plurality of microcells.
Cellular radio systems divide the area over which service is to be offered into a number of smaller areas called cells, each of which is served from its own base site, as previously mentioned. Each cell has its own antenna or antennas for transmission to and reception from the mobile station. An important principle of cellular radio is that frequencies in the system may be reused by several cells, provided that the geographic separation of the cells is sufficient to ensure that these common frequencies do not interfere with each other. This allows efficient use of the frequency spectrum available to the owner of the cellular system.
As a mobile moves from the service area of one cell into the service area of another, it must be instructed to communicate with the new cell and cease communication with the old cell. This process is known as "handover".
In mobile communication systems, the handover process relies on measurements made by the mobile station, the serving base station, or surrounding base stations. One important class of handovers, commonly referred to as "power budget handovers", involves making measurements on the radio link of the combination (mobile station and serving cell base station) in progress, supplemented by measurements which predict whether or not better quality would be enjoyed if the mobile were to be served by another base station. These supplementary measurements could be made by the mobile station on other base stations, or by the other base stations on the mobile station. In any event, when the measurements indicate that a better radio link would exist if the mobile were to handover to another base station, the handover to such base station is initiated.
As the traffic capacity requirements of multicellular radio network increase, cell sizes may be reduced to increase the density of traffic carrying channels per unit area within a fixed spectrum. However, users are diverse and some are better served by larger cells. Additionally, traffic density is not uniform and while some areas may require very small cells others do not. For these reasons, multi-cellular networks consisting of a variety of cells of different sizes arranged in layers superpositioned on top of one another as previously discussed have been proposed.
The speed and trajectory of a mobile and the size of a cell determine the time taken to traverse that cell. Thus, when cells in different layers are of different sized, the speed of the mobile becomes an important factor for determining which layer of cells a mobile would be best served by. What is important here is not the absolute speed but the speed measured in units of cells/unit time.
It is apparent, therefore, that optimum performance in such a network might best be achieved by segregating mobiles on the basis of speed as determined by the number of handovers they might suffer.
Mechanisms have been proposed which introduce a delay into the handover process so that mobiles travelling very fast with respect to the sizes of cell in the network layer by which they are being served, will prevented from staying in that layer. However, it is our view that this mechanism has serious disadvantages arising from the distortions to "best server" cell boundaries that it produces. In the extreme case this leads to the situation where a mobile will handover to a neighbouring microcell which it has already completely passed through. These distortions can lead to greater interference and an increased difficulty in planning a network.
Thus, there is a need for speed sensitive handover method which overcomes some of the main drawbacks of the simple delay based method.